templates: introduce revescape filter for escaping symbolic revisions There needs to be a way to escape symbolic revisions containing forward slashes, but urlescape filter doesn't escape slashes at all (in fact, it is used in places where forward slashes must be preserved). The filter considers @ to be safe just for bookmarks like @ and @default to look good in urls.

hgweb: allow symbolic revisions with forward slashes in urls It's possible to have a branch/tag/bookmark with all kinds of special characters, such as {}/\!?. While not very conveniently, symbolic revisions with such characters work from command line if user correctly quotes the characters. These characters also work in hgweb, when they are properly encoded, with one exception: '/' (forward slash, urlencoded as '%2F'), which was getting decoded before hgweb could parse it as a part of PATH_INFO. Because of that, hgweb was seeing it as any other forward slash, that is, as just another url parts separator. For example, if user wanted to see the content of dir/file at bookmark 'feature/eggs', url could be: '/file/feature%2Feggs/dir/file'. But hgweb tried to find a revision 'feature' and get contents of 'eggs/dir/file'. To fix this, let's assume forward slashes are doubly-urlencoded (%252F), so CGI/WSGI server decodes it into %2F. Then we can decode %2F in the revision part of the url into an actual '/' character. Making hgweb produce such urls will be done in the next 2 patches.